In numerous potassium deposits carnallite occurs together with sylvite or hard salt. The dressing of such carnallitic mixed salts requires special measures, which have for their goal the separation of the carnallite, either by itself or together with sylvite, from foreign minerals which do not contain potassium or which contain only small quantities of potassium.
For the separation of these mixed salts into the components thereof, electrostatic separating processes have been used. These processes depend upon the differences in conductivity of the components of the mixture and rest mainly on the finding that the film of alkali which forms on the surface of the mixed salt particles due to absorption of humidity from the ambient air develops on the carnallite crystals at higher temperatures and lower humdities than on the crystals of the other mixed components. For these processes, as described, for example, in the German Pat. Nos. 1,060,331 and 1,092,401, a previous chemical conditioning of the mixed salts is not required. However, these processes require a high expenditure in power and apparatus. For example, these processes require the use of roller separators since this type of separator will assure the necessary contact of the dressing material with an electrode. However with these separators a specific separator performance of at most only 0.5 ton of crude salt per meter of separator width an hour may be achieved.
Another development for the electrostatic separation of the components of the crude potassium salt uses the opposite electric frictional charge of the individual components to accomplish a separation in the electrostatic field. This phenomenon, as is well known, rests essentially on the fact, that in case of frictional contact of two bodies, the material which has the higher dielectric constant will be charged positively. Thus, the mixtures that are to be separated are moved in a fine dispersion across frictional surfaces made of substances having dielectric constants which lie between those of the components of the mixture that are to be separated. Mixtures treated in this manner may be separated in so-called free-fall separators as well as in roller separators. However, the separation effected by the pretreatment of the material to be separated is not satisfactory.
Therefore, the finding that the variable contact-electrical chargings of the components of the crude potassium salt is greatly improved by conditioning agents, and that certain temperatures and humidities of the atmosphere must be maintained during the electrostatic separation whenever the separation is accomplished in free-fall separators, represented an essential step forward.
Thus, for example, according to the process known from U.S. Pat. No. 3,217,876, the mixed salt that is to be separated is first treated, in a finely ground state, with an organic anionic surface conditioning agent in an amount of between about 0.01 and about 0.66 pounds of conditioning agent per ton of the mixed salt to be separated. The organic anionic conditioning agent may comprise, for example, carboxylic acids, their derivatives and their salts and mixtures, wherein the general formula of such carboxylic compounds is R--(COOR.sub.1).sub.n, where R is a hydrocarbon radical, R.sub.1 is hydrogen, a metal or hydrocarbon radical, and n is a whole number. The carnallite alone or mixed with sylvite is separated from this mixed salt by electrostatic separation at certain temperatures and certain relative atmospheric moisture.
Sulfonates or sulfates, which contain one or more SO.sub.3 Me or SO.sub.4 Me groups and at least 6 C-atoms in the molecule wherein Me is a metal ion, such as an alkali metal ion, have also been known as conditioning agents for these separations, for the preliminary treatment of the mixed salt. Still other conditioning agents are set forth in U.S. Pat. No. 3,217,876, such agents being incorporated herein by reference.
The sylvite-carnallite preconcentrates obtained by the above process must be separated still further for the production of technically pure products. For this, a process has been known, for example, from the U.S. Pat. No. 3,225,924, the disclosure of which is incorporated herein by reference. According to that patent, the sylvite-carnallite preconcentrate is conditioned with organic acids, such as, for example, benzoic, phthalic, salicylic, cinnamic, atropic, phenylacetic and vanillic acid, as well as with their salts or simple substitution products, or with nitrosonaphthols, or with mixtures of these substances.
These processes for the electrostatic separation of crude potassium salts based on contact-electric charging have the disadvantage that sylvite is likewise inclined increasingly toward a reversal of charge with increasing carnallite content of the crude potassium salt and is charged against carnallite. These facts are confirmed by the results of the experiments stated subsequently in example 1. These results, obtained by experiments carried out in accordance with the status of the prior art, show that it is not possible according to present knowledge to separate crude carnallitic salts with a carnallite content of over 10% by weight into a K.sub.2 O valuable substance fraction consisting of sylvite and carnallite by way of electrostatic separation, and to separate such crude salt into an abandonable residue which consists essentially of native magnesium sulfate and rock salt or of rock salt alone.
Therefore, prior to the present invention, there has been a need for a process for separating carnallitic crude potassium salts by means of electrostatic separating techniques into a K.sub.2 O valuable substance fraction and into an abandonable residue which consists essentially of a mixture of native magnesium sulfate and rock salt or of rock salt alone. According to the present invention, it has now been found that carnallite-containing ground, conditioned potassium salts can be separated into a K.sub.2 O valuable substance fraction and into an abandonable residue which consists essentially of a mixture of native magnesium sulfate and rock salt or rock salt alone by treating the carnallitic crude salts with air of 5 to 25% relative humidity in two steps by means of electrostatic high voltage fields. According to the present process, a carnallitic fraction is separated from the crude potassium salt in a first separator section of 0.4 to 1.2 m field length (falling section) and the middlings which had become deficient or depleted in carnallite are delivered to a second separator section of 1.5 to 2.5 m field length (falling section) and are there separated into a sylvite preconcentrate, a middling and a residue deficient in valuable substances.
Advantageously the middling obtained in the second separator section is recirculated in this second separator section.
In order to carry out the process of the invention, carnallitic crude potassium salts are used which have advantageously been ground to an average particle size of 0.5 to 1.5 mm. As is well known, these carnallitic crude potassium salts contain a considerable quantity of carnallite and, in addition, rock salt and possible native magnesium sulfate.
After grinding, the crude potassium salt is conditioned in a manner known per se, for example, in the manner set forth in U.S. Pat. Nos. 3,217,876 and 3,225,924. For this, the conditioning agents which have been proposed in U.S. Pat. Nos. 3,217,876 and 3,225,924, and which have been discussed above, may be used in the quantities of generally about 0.01 to 0.66 pounds of conditioning agent per ton of mixed salt, more or less being dependent on the particular source of crude salt. Preferably, these conditioning agents are used in quantities of 5-300 g/t crude potassium salt. The conditioning agent actually is mixed as a solution or emulsion with the crude potassium salt to be dressed in such a way, that the conditioning agent is distributed in the crude potassium salt as homogenously as possible. For this purpose, water generally is suitable as the solvent or emulsifier, and an aqueous solution containing, for example, 2.5% by weight of the conditioning agent has been found to be satisfactory. However, in place of water, an alcohol such as methanol, ethanol, or isopropanol may be used as the solvent. Other solvents or emulsifiers may be selected as a matter of choice depending upon the particular conditioning agent employed. Any solvents or emulsifiers which are used during the conditioning are to be removed prior to the introduction of the conditioned crude potassium salt into the electrostatic separator. This may be accomplished, for example, by blowing a current of heated air (100.degree.-175.degree. F.) through the conditioned salt mixture while stirring and mixing the same. It will be appreciated that the portion of the crude potassium salt which is added in the second separator step may be so added without any additional conditioning being performed. However, if so desired, that portion of the crude salt which is added in the second separator step may be conditioned, for example, in the same manner and with the same conditioning agent as was the case for the original conditioning. However, the conditioning may be changed by the application of a conditioning agent which is different from the agent used in the first step.
After the first conditioning, the finely ground crude potassium salts are adjusted to a relative humidity between 5 and 25% in a conventional manner. For example, the salts may be adjusted in an apparatus as described, for example, in German Pat. No. 1,283,771, to ascertain relative humidity, which may be between 5 and 25% and the most favorable value of which for the pertinent separating material may easily be determined by simple preliminary experiments. Possibly the crude potassium salt, in case of this adjustment to a certain relative humidity, may be preheated to the temperature at which the electrostatic separation of the components is to take place. Advantageously this temperature is between 20.degree. and 80.degree. C. However, it may also be up to 200.degree. C.
The conditioned crude potassium salt, adjusted to a certain moisture content and possible to a certain temperature, is then inserted in a high voltage field of an electrostatic separator. As can be appreciated, the downward velocity of the material at the point of entry into this field is negligible when compared to the average velocity of the material in the field. The average velocity is determined by the force of gravity acting on the material as well as the sidewards force of deflection generated by the field. In this case, the high voltage field is to be developed by proper selection of the electrodes and their size, such that the falling distance of the material fed into the separator through the high voltage field amounts to 0.4 to 1.2 m. Since the carnallite has a high specific surface charge, it is very much deflected in the high voltage field and is to be found greatly enriched immediately adjacent to the positive electrode of the separator. As can be appreciated from FIG. 2, and as will be described later, as the downward velocity of the material due to gravity increases, there is a significantly less deflection of the material. Thus, it is clear that the entry velocity of the material into the field is negligible when compared to the average velocity imparted to the material, travelling through the field, by the combined force of gravity and the force generated by the electric field. By a correspondingly arranged discharge arrangement, a fraction, greatly enriched with carnallite may thus be removed from the separator as a carnallite preconcentrate and may be fed to an additional processing, for example, to a separate after-purification. The fraction obtained as a middling, thereby becomes so deficient or depleted in carnallite, that said fraction can be fed to an additional electrostatic purification to recover sylvite.
For this purpose, this middling fraction is fed to a second separator wherein the distance through which the material will fall through an electrostatic high voltage field amounts to 1.5 to 2.5 m. In this high voltage field, the sylvite is deflected to the positive electrode and may be removed from the separator by means of a properly disposed discharge arrangement as a sylvite preconcentrate with an output for sylvite of about 70%. The middling discharging from the second separator by means of a second discharge arrangement is preferably circulated through the second separator. In addition, a residual fraction may be removed from the second separator by means of a third discharge arrangement located near the negative electrode, the content of valuable substance of which is so small that it may simply be dismissed.
Both the carnallite as well as the sylvite preconcentrate may be processed in an additional electrostatic separation in separators of traditional construction into concentrates with over 90% carnallite or sylvite content, whereby the native magnesium sulfate, contained in both concentrates, remains in the residue. However, there also is the possibility to process these preconcentrates separately or jointly to high percentage potassium fertilizer salts and magnesium sulfate.
In the process of the invention, the losses of valuable substance amount to only about 6% for K.sub.2 O and only about 5 to 6% for native magnesium sulfate, and they thus are extremely small. According to the process of the invention, crude potassium salts, especially with high carnallite content, may thus be processed in one operation to preconcentrates, without the occurence of opposite charging of the sylvite, as compared to carnallite, which could always be observed in prior processes for the electrostatic separation of such salt mixtures. Also, as a residue, there remains merely a product which because of its small content of valuable substance, may be eliminated from the process.